The present invention relates to ion exchange polymer films and to processes and apparatus using these films.
Fluorinated ion exchange polymers are known in the prior art formed from an intermediate or precursor polymer which contains pendant side chains with sulfonyl fluoride groups. These groups are converted to ion exchange sites such as by hydrolysis with an alkaline material or by treatment with ammonia. An example of such prior art teaching is disclosed in U.S. Pat. No. 3,282,875 to Connolly et al.
Treating cation exchange polymers to modify relative cationic transport is disclosed in U.S. Pat. No. 3,647,086 to Mizutani et al. for preparation of an ion exchange membrane in which permeation selection of different classes of cations is improved. As set forth in this patent, a cation exchange polymer of a high molecular weight polymer contains chemically bonded acid amide groups. These groups are present at the substantial surface portion to satisfy the equation: EQU /A (A + B) .times. 100 = 15 - 10.sup.-5 %
wherein (per gram of dry membrane) A is the number of acid amide bonds and B is the number of cation exchange groups. The reaction is controlled such that the formation of the acid amide bonds takes place only at the surface or as set forth in the patent at the "substantial surface".
An important utility for ion exchange polymers is in the electrolysis of an aqueous solution of an alkali or alkaline earth metal halide to produce hydrogen and the corresponding halogen and metal hydroxide. By far the most important application in this field is the chloralkali industry using aqueous sodium chloride as a source material.
A need has developed in the chlor-alkali industry for the use of improved ion exchange materials which can replace existing separators in an electrolytic cell which have been used for decades without substantial improvement in design.
In the environment of a chlor-alkali cell, the ion exchange polymer must be able to withstand a hostile environment such as exposure to a highly alkaline pH as well as exposure to chlorine and concentrated brine at temperatures approaching 100.degree. C. Generally, hydrocarbon ion exchange membranes are totally unsatisfactory for this application since the membrane cannot withstand this environment.
For commercial usage in the chlor-alkali industry, a separator formed from an ion exchange polymer must go beyond the ability to be operable for prolonged time periods in the production of chlorine and caustic soda. An important criteria for the polymer is the ability to operate an electrolytic cell with high current efficiency and with low levels of impurities resulting from undesirable by-products. Also, an improvement in current efficiency can translate into pronounced savings in the cost of production of each unit of chlorine and caustic, e.g. a reduction in power consumption. The operation of the electrolytic cell with the proper separator to produce concentrated caustic soda is highly desirable to reduce the expense of production.